1. Field of the Invention
The present invention relate to epitaxial growth technique, more particularly, to a method of epitaxially growing a group III-V compound semiconductor crystal using the group V element hydride and the group III element halide and a molecular beam epitaxy apparatus for the same.
2. Description of Related Art
In a conventional chemical vapor deposition (CVD) of the group III-V compound semiconductor such as GaAs using the group III element halide and the group V hydride, species to be grown on the group III elements are the group V element cluster molecules such as As.sub.4 and AsH.sub.2 produced by decomposing the group V element hydride such as AsH.sub.3 under a growth pressure condition. The active surface of the group III elements has been produced by taking halogen atoms out of the group III element halide by use of an H.sub.2 gas ambient. In this halogen-removal reaction the activation energy of 40 to 50 kcal/mol is required and the reaction is just a rate-determining step over the whole of growth cycles.
In another CVD the group III element halide such as GaCl and InCl is adsorbed on a substrate in the not-decomposed state by reducing the collision between the group V element hydride molecules under a low pressure or by increasing the flow rate of the molecular beam of the group V element hydride and the group V element hydride is supplied to the surface of the substrate such that the group V element molecules directly combine with the group III element molecules. As a result, the growth rate is higher than a conventional growth rate in the order of 10.sup.3 to 10.sup.4 and is not dependent upon the H.sub.2 pressure in the reaction ambient. This is reported with respect to AsH.sub.3 by Gruter et al. in Journal of Crystal Growth (94, (1989), 607) and with respect to PH.sub.3 by Karlicek et al. in Journal of Electrochemical Society (134, (1987), 470) and by Beccard et al. in Journal of Crystal growth (121, (1992), 373). That is, it could be considered in these cases that the reaction of production of hydrogen halide molecules of halogen atoms and hydrogen atoms in the ambient H.sub.2 gas and then the reaction of desorption thereof from the substrate do not occur. On the contrary, it could be considered that the group III atom on the surface of the substrate directly combines with the group V atom in the ambient through another reaction path. According to these reports, the growth rate is improved and the film thus obtained is superior in electrical properties such as Hall mobility. However, about 1000 K. is expected as a temperature condition of direct combination reaction between the group V element hydride and the group III element halide. Therefore, there is the demand for a lower temperature condition.
Further, atomic layer epitaxy (to be referred to as "ALE" hereinafter) is well known in the art in which halides are used as the group III medium. In conventional halide ALE, the group V cluster molecules produced with thermal decomposition of the group V element hydride react with the active group III atoms on the substrate surface which are produced by taking halogen atoms out of the substrate surface with H.sub.2 in the ambient which is a rate-determining step, so that the growth of the group V atoms on the group III atoms is performed. This reaction processes are similar to the above conventional reactions processes.
The ALE is used to form a quantum wave effect device such as a quantum wire device and a quantum well device which requires the fine structure of a nano-meter scale. If the ALE is used, therefor, it is possible to uniformly grow a film on a fine well or groove structure formed in a substrate. However, in order to avoid the deterioration of the nano-meter scale structure to be produced because diffusion is not desired, the growth temperature should be typically set at 700.degree. C. or below.
Thus, even in the ALE for the halide, it is desirable that the group III atoms are directly combined with the group V atoms to increase the overall growth rate by suppressing the decomposition of molecules of group V element hydride to control the supply to the substrate and by avoiding the removal of halogen atoms by use of H.sub.2 gas as the rate determining step, and a film having good electrical properties can be obtained by suppressing the incorporation of impurities into the film and the production of defect. However, in the conventional direct reacting method of the group V element hydride and the group III element halide, the low temperature condition for growth required in the ALE cannot be established.
Further, in the conventional ALE, the selective growth on only a specifically oriented surface plane among adjacent surfaces is difficult.